1. Field of the Invention
This invention relates to writing liquids for writing on opaque recording materials to form transparent lines against an opaque background, as well as to unique combinations of writing liquid and opaque recording material for forming transparencies to be used with overhead projection machines and the like as well as to a method of writing in the form of transparent lines on an opaque recording material.
2. Discussion of the Prior Art
U.S. Pat. No. 3,014,301 to W. F. Grupe relates to a recording material, referred to as a recording chart or chart medium, for use in recording units with a heated or a pressure stylus, and/or a pen containing a solvent. The chart medium is made with a transparent film backing upon which is deposited a white, opaque coating which is heat, pressure and solvent sensitive. The coating is formed from an unstable solution of nitrocellulose in acetone (a solvent) and xylol (a non-solvent). The whiteness and opacity results from the acetone evaporating first leaving a high percentage of xylol and nitrocellulose in solution, from which the nitrocellulose is precipitated as a white solid. The chart is transparentized by "printing" the opaque layer with a solvent such as Cellosolve or carbitol acetate from a solvent dispensing pen. A transparent dye can be added to the solvent to form transparent lines color dyed but photographically transparent so that it may be used as a negative for photographic reproduction or as a positive transparency for projection onto screens or other medium. The chart medium can be used, for example, for preparing graphs with conventional pen-operated machines.
A similar sheet recording material which is sensitive to pressure is disclosed by R. S. Ives in U.S. Pat. No. 2,962,382, except that water replaces xylol as the high boiling non-solvent and various film-forming polymeric materials such as cellulose acetate, ethyl cellulose and polyethyl methacrylate are disclosed in addition to nitrocellulose as the opaque material. This patent also discloses methyl ethyl ketone and methyl alcohol, in addition to acetone as suitable volatile solvents for the polymeric materials. However, the opaque coatings taught by Ives require large amounts of surfactant or a combination of surfactant and waxy lubricant to produce useful pressure-sensitive materials.
While the recording materials described in the above patents provide satisfactory transparencies in many cases they suffer from the drawback that it is very difficult to obtain uniform opaque coatings by depositing a layer of polymeric material from an unstable mixture of volatile solvent and high-boiling non-solvent. Precise control of the rate of evaporation of the volatile solvent is necessary to obtain opaque coatings and uniformity of the opacity over the entire surface. Reproducibility from sheet to sheet is also difficult. Furthermore, the degree of opacity with the polymer/solvent/non-solvent systems of these patents is generally not as high as desireable for good contrast between the transparent film backing sheet and the opaque coating layer.
A heat sensitive recording sheet material of improved properties, including opacity, has been sold by the Minnesota Mining and Manufacturing (3M) Co. under the designation Thermofax Transparency #528. As the name suggests, this sheet material is sold for use for making transparencies with a Thermofax infrared recording machine and includes an opaque layer of a styrene resin, acrylic resin and silica on a transparent polystyrene backing sheet. There is no indication by the 3M Co. that this recording material can be transparentized with a solvent for the styrene resin.